Different retroviruses have been isolated from various mammalian species by co-cultivation of tisues with heterologous cells. These isolates include five different primate viruses: type C viruses from two Old World monkeys (baboon and macaque) and from a New World primate (owl monkey); and type D viruses from Old World (langur) and New World (squirrel monkey) primates. These isolates are all endogenous in their respective species, and are present in multiple copies in the cellular DNA of these and related species. Molecular hybridization studies employing radioactively-labeled DNA probes from these five primate virus groups reveal that these viruses are essentially unique, although they may share a small degree of more distant homologies with each other and with certain mammalian viruses. Nucleic acid sequences related to these viruses are present in a wide variety of primate species. The development of various hybrid cell lines (rodent and primate) that preferentially segregate primate chromosomes will allow the mapping of these endogenous viruses to specific primate chromosomes.